Known apparatus have certain problems near hovering flight because the sensors typically used are not very accurate at the low dynamic pressures associated with the low speeds to be measured.
In addition, prior sensors do not permit easy measurement of speed oriented in any direction, particularly toward the rear or laterally, which is one of the principal advantages of the helicopter.
In addition to dynamic pressure sensors, French Pat. No. 1567 128 suggests installing in the airflow of the rotor a system with two yaw meters oriented respectively along the longitudinal and transverse axes of the helicopter.
That device, which is generally effective, nonetheless has some limitations due to its external location and to its relative fragility which requires frequent maintenance and which is too burdensome for a mass-produced helicopter.
French Pat. No. 2282644 (which corresponds to U.S. Pat. No. 4,203,409) relates to a device comprising two measuring devices (one for the longitudinal and the other for the transverse speed component) wherein each measuring device includes both a position detector for cyclic rotor pitch control along the axis concerned, and a detector for the acceleration component along the same axis, as well as related measuring implements to provide integration of the algebraic sum of the values measured by the two corresponding detectors.
This device functions very well and with remarkable precision, but nonetheless remains relatively expensive, which excludes its use for low-priced or mass-produced helicopters.